Coating for cadmium and zinc



Patented Jan. 29, 1946 UNITED STATE s v PATENT orrlcr COATING FOR CADMIUMI AND ZINC Robert M. Thomas and Charles W. Ostrander, h Baltimore, Md., assignors, by memo assigm ments, to Rheem Research Products Income-- rated, Baltimore 17, Md., a corporation of New York No Drawing. Application March. 22, 1943,

' Serial No. 480,096

Claims. (Cl. 148-6) This invention relates to the treatmentof zinc and cadmium and particularly galvanized metal to produce thereon an. insoluble adherent corrosion resistant coating which is visible and may acid,v alone do not produce a satisfactory surface,

when such compounds are combined in a dip, it very satisfactory corrosion resistant finish is obtained according to this invention. Thus we use chromic acid, chromates, and dichromates with formic acid or formates, and dependent upon the relation of the ingredients in the dips, e. g., as described in the examples set forth below, obtain corroiion resistant coatings of various shades which are highly desirable. The temperatures and time periods of treatment may also be varied to speed up as well as modify the coatings. One particular coating has a distinctly olive-drab ap-- pearance and meets the demand for a color of this character in which the coating is corrosionresistant under such conditions as those encountered by contact with salt water, and in tropical climates.

In order to smooth or polish as well as vary the color and enhance corrosion resistance of the coatings, we include in the dip compounds which will liberate anions of mineral acids. Thus, we may include in the dip mineral acids in amounts effective for the purpose of brightening the coating, or. we may include salts of the mineral acids for the purpose.

The dip reacts with the zinc and in the case of galvanized metal forms the coating on the surface of. the relatively thin surface layer of spelter as a molecular film. i. e., combines with the zinc spelter layer to produce thevisible corrosion resistant coating. We attribute the effectiveness of thedip, at least in part, to the reduction in valence of the chromium which takes place in the presence of formic acid or a formate, and the action of thelformic acid or formate in maintaining the oxidation reduction reaction between the base metal, e. 8.. zinc, and the chromic ion atthe optimum for obtaining a favorable deposit.

' celerate the deposit and, hence, reduces the time This opinion is based upon observation and is simply our explanation of what We consider occurs. Having experimented with chromate dips including chromium compounds and. mineral acids as well as organic acids, we can safely say that through a wide range of proportions, temperatures and immersion periods, the present invention provides a reliable coating which has markedly improved corrosion resistance.

In order that the invention may be more clearly understood, we will describe several satisfactory examples of dips:

' Example I In this' example, substantially 100 grams of chromic acid per liter and substantially 60 cc. of

- formic acid per liter are employed. An immersion of about 15 seconds to about a minute and a half or about 2 minutes may be used, and the temperature may vary from about room temperature to about boiling. Heating appears to acperiod of immersion required.

Example II In this example about 60 co. per liter of formic acid and about grams per liter of chromlc acid are. contained in the dip. The temperatures and time periods are substantially the same as in mample I.

Example III In this example, soluble chromates such as potassium or sodium chromate or dichromates, such Gil as sodium or potassium dichromates, are'employed and substituted for the chromic acid of Example I or II. We find that satisfactory coatings are obtained if the chromates and di-chromates'are used in substantially twice the amount of the chromic acid in Example I or II, the proportions, as well as time oi immersion and temperature being about the sadness in the two previous examples. Example IV In each of the coatings described in Examples I u to'IV a mineral acid. namely, hydrochloric, sol- 40 cc. per liter to about 100 cc. per liter.

in the claims as "anions of a mineral acid,"-it

furic or nitric, or a salt of the mineral acid, such as cobalt nitrate, zinc nitrate, zinc chloride, sodium chloride, copper sulphate or ferric sulphate,

are added in amount suiificient to render the coating produced by the-reaction of the chromium compound and the formic compound smooth and polished. The mineral acid or salt of mineral acid enhances corrosion resistance and acts as a color varying agent.

An article having a surface of zinc or cadmium orgalvanized metal or containing these materials, when treated in accordance with Example V has a coating comprising a plurality of identifiable superposed films. That is, the coating comprises a water insoluble interfacial film composed essentially of an oxide' of the base metal, which film is integrally united with the base metal surface and bonds thereto a second superposed visible film composed essentially of compounds containing trivalent and hexavalent' chromium in which the water insoluble trivalent compounds are in greater amount than the hexavalent compounds.

It'will be observed from the foregoing that either the chromium compound or the formic compound may be used in greater or less amount depending upon the coating, desired. In all cases a visible surface coating is produced which, as explained above, has substantially improved corrosionresistant properties.

In preparing suitable dips, the chromic acid may be present in amounts from about 15 grams per liter to about 150 grams per liter, and the formic acid may be used in amounts from about The mineral acid will be used in amount of about 5 to cc. per liter for smoothing and brightening purposes and larger amounts are employed where a formate is employed to liberate the formic acid, e. g., up to about 50 cc. per liter. As explained above, where chromates or di-chromates are used, the amount is substantially twice the chromic acid. Where salts of mineral acids are used,"

from 1 to 50 grams per liter have beenfound satisfactory. Also, as explained above, where soluble formates are employed instead of formic acid,

they are-used in substantially the same amounts as formic acid. In other words, the chromium compounds are present in amounts equivalent to to 150 grams per liter of chromic acid, and the organic compound is present in amount equivalent to 40 cc. to 100 cc. per liter of formic acid, whilethe mineral acidsalts are used in amounts equivalent to 5 to 10 cc. per liter of the mineral acid, and larger amounts are used where a formate is employed as explained above.

well as salts of mineral acids other than those given as examples may be utilized provided they are soluble, and also in the case where formates are used will act to liberate formic acid.

In referring to cadmium and zinc" in the claims, we intend to include by this expression galvanized metal whether prepared by plating or hot-dipped and in which cadmium or zinc or mixtures thereof are employed.

Likewise, the claims are intended to cover the use of mixtures of the chromium compounds, mixtures of the formic compounds, mixtures of the mineral acids, mixtures of the salts of mineral acids as well as mixtures of such acids and salts.

Preferably, the coatings described herein are obtained by simply immersing the zinc or cadmium, or galvanized metal to be coated in the solution until the desired coating is formed. This will vary with changes in concentration, temperature, time of immersion and salts or acids used. The coated object is rinsed in hot or cold water to obtain an even finish or allowed to dry without rinsing at which time an attractive mottled appearing coating will be formed. Pretreatment other than cleansing is not necessary in coating zinc or cadmium or galvanized metal.

No special rinse is necessary to increase the corrosion resistance. o

Referring to Example IV wherein a formate is substituted for formic acid and a mineral acid is employed for the purpose of liberating formic acid, suflicient mineral acid is employed to bring the pH of the solution to about pH 0.5 to about pH 2.5. The pH will necessarily vary in the preparation of dips in accordance with Example IV depending upon the constituents employed and their proportions.

As will be observed, the invention may be car.- ried out in a very simple. manner and one which we find can be utilized for continuous treatment to reliably give a uniform product at low cost. While corrosion resistance is the most important consideration, the etched metal is provided I intended in the appended claims to mean the particular constituents recited inamounts effective to produce a visible corrosion resistant. coating on zinc and cadmium when subjected to the solution, but does not exclude the use in the solu- As indicated in the examples, the temperature 1 may vary from substantially room temperature to about boiling, and the time periodof immersion from about 15 seconds to substantially a minute and a half or 2 minutes or until a visible coating having the desired properties is formed.

In the claims, we refer to a chromium compound" and by that term intend to include not only chromic acid but they chromates and d1- chromates as well, it being understood that other soluble chromates and di-chromates than mentioned herein are susceptible of use in accordance with the present invention. It is to be understood that like the chromium compounds, soluble formates other than recited herein may be utilized. In referring to the mineral acids and salts of mineral acids, we have described these being understood that numerous mineral acids I tion of other'components where the latter do not substantially alter the basic character of the coating obtained by use of the essential components recited. i i

We claim:

l; A method of producing an adherent corrosion resistant coating on zinc and cadmium, comprising subjecting the same to an aqueous acidic solution essentially consisting of a water soluble chromium compound selectedfrom the group consisting of chromic acid and it: salts and an organic compound which is easily oxidized by chromic acid and selected from the group con-. sisting of formicacid and soluble formates until a visible coating is formed, the amount of the organic compound being suiflcient to cause the chromium compound in conjunction with the organic compound to produce a visible corrosion r6- sistant. chmmimn-mntaiuimcoatins on flie base memo 2.- A- method-of prodming an Wentcomsion resistant coating on zinc and cadmimn, com

accuses the chromium compound in conjunction with the organic compound to produce a visible corrosion resistant chromium containing coating ,on the base metal.

3. A method of producing an adherent corrosion resistant coating on zinc and cadmium comprising subjecting the same to an aqueous acidic solution essentially consisting of a water soluble chromium compound selected Irom the group consisting of chromic acid and salts thereof and an organic compound which is easily oxidized by chromic acid and selected from the group consisting of formic acid and soluble tormates and anions or another mineral acid, until a visible coating is formed, the amount of the organic compound being suilicient to cause the chromium compound in conjunction with the organic com pound to produce a visible corrosion resistant chromium containing coating on the base metal.

. 4. A method in accordance with claim 3 wherein the metal is subjected to the dip for about fiiteen seconds to about two minutes.

5. A method in accordance with claim 3 wherein the metal is subjected to the dip at a temperatgre from about room temperature to about 21 l8.

6. An aqueous acidic solution for producing an adherent corrosion resistant coating on zinc and cadmium consisting essentially of a water soluble chromium compound selected from the group consisting of chromic acid and salts thereof and an oroonic compound which is easily oxidized by chromic acid and selected irom the group conoistinc or iormic acid and soluble iormates, said chromium compound and said organic compound heino present in cunts effective to produce a chromium containing visible coating on zinc and cadmium subjected to the solution, untlthe or conic compound beine used in amount creator than the equivalent or about to corner liter oi tormio acid.

of the formic acid being sumcient to cause the chromium compound in comunction with the formic acid and zinc nitrate to produce a visible corrosion' resistant chromium contaimne coating on the base metal.

9. A method of producing an erent corrosion resistant coating "on zinc and cadmium comprising subjecting the same to the aqueous acidic solution essentially consisting of sodium dichromate, formic acid and zinc nitrate, until a chromium containing visible coating in formed, the amount of the formic acid being cient to cause the chromium compound in conjunction with the formic acid and zinc nitrate to produce a visible corrosion resistant chromium con coating on the base metal.

10. An article having a surface or zinc and cadmium provided with a coating in accordance with claim 3, said coating comprising a water insoluble interfacial film composed essentially of an oxide of the base metal, said film being in.- tegrally united with the base metal surface and bonding thereto a second visible film composed essentially of compounds containing trivalent and hexavaient chromium in which the water insoluble trivalent compounds are in greater amount than the hexavalent compounds. 3

11. An article having a surface of zinc and cadmium provided with a coating in accordance with claim 9, said coating comprising a water insoluble interracial film composed essentially otan oxide of the base metal, said being integrally united with the base metal surface and bonding thereto a second visible film composed essentially oi compounds containing trivalent and heuovalent chromium. in which the water insoluble trivalent 7 compounds are in greater amount than the hexovalent compounds.

' in. An aqueous acidic solution for producing an adherent corrosion iresistant coating on zinc and cadmium consisting essentially of a water soluble chromatoand an organic compound which is easily oxidized by chromic acid and selected from the group consisting of formic acid and soluhle iormotes, the amount oi the organic comporicl beine sumcient to cause-the chrourn compound in conjunction with the organic compound to produce a visible corrosion resistant chromium I contain coating on the hose metal.

ct another mineral acid, th amount oi the or s anic compound ioeinc sufidcient to cause the" chromium compound inconiunctiori with theor eanic compound and said mineral acid lo to produce a visible corrosion resistant chromi containing coating on the base metal.

8. aqueous acidic solution for producing an adherent corrosion resistant cootine on t. and

' cadmium consisting essentially oi sodium clichromate, formic acid and zinc nitrate, the at it. An auueous acidic solution tor producing on adherent corrosion resistant coating on aim: and cadmium coneistiire essentially of water scluiole oilchrcmete and on organic compound which is easily oxidized by chromic acid and selected from the troop consisting oi formic acid and soluble rormates, the amount of the organic compound being scient to cause the chromium compound in conjunction with the organic compound to produce a visible corrosion resistant chromium conto r coating on the lease metal.

it. ocueous acidic solution according to unions oi a mineral acid. it. An aqueous, acidic solution according to claim 12 also claim 12 also contg anionsot a mineral acid.

Certificate of Correction Patent No. 2,393,663. January 29,1946.

' ROBERT. M. THOMAS ET AL.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 3, second column, line 64, claim 15, for the claim reference numeral 12 read 18; and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Ofiic.

Signed and sealed this 3rd day of September, A. D. 1946.

LESLIE FRAZER, 1

First Assistant O'ommisaianer of Patents. 

